As transistor and other integrated circuit (IC) dimensions shrink, the circuits become more susceptible to the effects of ionizing radiation. For example, capacitors in DRAMs may be discharged, cells in SRAMs may change state and other deleterious effects may occur as a result of stray charge and ionizing radiation from radioactive materials or from cosmic rays.
In addition, charge released by an ionizing particle may be trapped in an oxide or nitride layer and increase leakage across a junction, or along a path that nominally blocks charge flow. For example, charge trapped in the gate oxide of an NMOS transistor may decrease the threshold of the transistor and thus increase the leakage between source and drain in the off state, as well as changing the time during a voltage ramp when it turns on. Charge trapped in the field oxide surrounding the device may form a leakage path along the vertical edge of the transistor body between the source and drain (S/D). The overall effects of radiation-induced damage can result in spurious circuit operation or even in non-functional devices on an integrated circuit.
Special hardening techniques, such as field hardening, have been developed in an attempt to address these problems. However, these processes are non-standard and are not readily integrated into a conventional IC fabrication line, and thus their use incurs substantial additional cost and complexity, especially for relatively low volume production runs (e.g., some hundreds or a few thousand ICs).